Add enum type in mappings

[rbino]

It would be nice to have a datatype that guarantees that the device/server can only send a value picked from a finite set of valid values, discarding invalid ones. This could potentially be backed by an integer type in the BSON format, making the on-wire payload compact (expanding it to a string on the device/server side).

Currently we use string types for this and we document the possible values in the description, but this leaves everything up to the application (and doesn't prevent invalid values to pass through Astarte and be saved to the database).